1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a nuclear fuel assembly bottom nozzle to control rod guide thimble attachment system which allows for bottom nozzle fuel assembly reconstitution.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission. The reactor also has control rods which can be inserted into the guide thimbles to control the fission reaction. The fission reaction releases a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
During operation in the nuclear reactor, the fuel rods may occasionally develop cracks along their lengths resulting primarily from internal stresses. These defective fuel rods must be replaced in the fuel assemblies, and this replacement must occur under water as the fuel assemblies become highly radioactive during their operation in the reactor. To gain access to a defective fuel rod, it is necessary to remove the top and/or bottom nozzle of the fuel assembly. Reconstitutable fuel assemblies exist which are designed with removable nozzles. Typical removable top nozzles have been attached to the top of the guide thimbles using a threaded or bulge/groove arrangement. Typical removable bottom nozzles have been attached to the bottom of the guide thimbles using a threaded arrangement. Heretofore, bottom nozzle removal and replacement has required inverting the fuel assembly, such as disclosed in U.S. Pat. No. 4,522,780, hereby incorporated by reference. There is a concern that inverting an irradiated fuel assembly may damage the fuel pellets in the fuel rods.